Although more than 1 in 4 men can be expected to develop symptomatic inguinal hernia, its mechanism is currently unknown. A subset of hernias may develop due to muscle fibrosis and myofiber atrophy leading to lower abdominal wall weakness. The long-term objective of this application is to determine the role of estrogen action in the etiology of lower abdominal muscle tissue (LAMT) fibrosis and atrophy associated with a subset of inguinal hernias. Aromatase, which converts testosterone to estradiol, is expressed only in the brain and testes of male mice. However, in men, aromatase is expressed in many additional tissues (muscle, fat) to provide physiologically necessary local quantities of estrogen. We generated transgenic humanized aromatase (Aromhum) mouse lines, each containing a single copy of the full-length human aromatase gene including its regulatory region, to mimic human patterns of estrogen production. Aromhum mice express the aromatase gene in peripheral tissues including the fibroblast component of the skeletal muscle tissue. LAMT has been found to be more sensitive to estradiol than the upper abdominal or quadriceps muscles, because the stroma of LAMT contains strikingly larger amounts of estrogen receptor-? (ER?)-expressing fibroblasts. Locally increased concentrations of estradiol in LAMT was associated with LAMT fibrosis characterized by progressive replacement of atrophic myocytes (muscle fibers) with ER?-rich fibroblasts and excessive extracellular matrix, resulting in formation of large inguinal hernias in >90% of Aromhum male mice by 24 weeks. However, there were no hernias observed in any of the wild-type (WT) littermates. Microarray expression analysis of LAMT at four weeks (before the appearance of hernias) showed activated profibrotic pathways in Aromhum vs. WT mice. We hypothesize that enhanced estrogen action caused by locally formed estradiol drives muscle fibrosis and myocyte atrophy, leading to the hernia phenotype affecting highly estrogen-sensitive portions of skeletal muscle tissue, which is LAMT in Aromhum mice. This resonates with the remarkable and parallel increases in inguinal hernia incidence and increased aromatase expression in skeletal muscle and fat in aging men. To ascertain the underlying mechanisms, we propose the following aims: 1. Determine whether treatment with an aromatase inhibitor, an estradiol antagonist, or a highly selective ER? antagonist prevents fibrosis, LAMT muscle atrophy, and hernia formation in Aromhum mice. The estradiol/ER?-mediated genomic mechanisms responsible for disordered proliferation of fibroblasts and extracellular matrix formation will be determined using integrative analysis of RNA-seq and ER?-ChIP-seq on LAMT and fibroblasts. 2. Determine whether the genetic disruption of ER? selectively in skeletal muscle fibroblasts affects LAMT fibrosis and hernia formation in Aromhum mice. In parallel, we will assess tissue steroid levels, aromatase and ER??expression, and estrogen responsive genes in abdominal muscle biopsies of men with or without hernia. We anticipate that this novel proposal will identify new drug targets and likely lead to the discovery of preventive approaches for hernia in high-risk populations.